Process of impact coating solid insulators with transparent conductive coatings



April 9, 1957 2,788,297

. A. S. LOUIS PROCESS OF IMPACT COATING SOLID INSULATORS. WITH TRANSPARENT CONDUCTIVE COATINGS led NOV. 15, 1951 FIZZ/-1- v INVENTOR. IRNOLD 5. Laurs il mk A? HE NT PROQESS OF IMPACT COATING SOLID INSULA- TORS WITH TRANSPARENT CONDUCTIVE COATINGS Arnold S. Louis, New York, N. Y., assignor to Myron A. Coler, Scarsdale, N. Y.

Application November 15, 1951, Serial No. 256,516

12 Claims. (Cl. 117-211) This invention relates to electrical insulators having electrically conductive surfaces and method of making same.

In many applications for electrical insulators such as glass and plastics it is desirable that an electrically conductive surface be provided in order to eliminate electrostatic charges. In electrical measuring apparatus electrostatic charges cause disturbances to sensitive meters. Accordingly, a conductive surface is sought for the meter casing observation window. In another typical case, that of aircraft, the electrostatic charging of the plastic cockpit canopy and insulating plastic surfaces covering radio and radar antennas create electrical interference with radio communication and radar signals.

Again, it is often desirable that a coating of considerable conductivity, but not usually transparent, be applied to an insulator to serve as the base for electroplating. Yet again, a firmly adherent, highly uniform coating of conductive material on an insulator sheet can serve as an electrical plotting board or for related electronic applications. Furthermore, a conductive coating applied to an insulator surface can be used as a resistive heater element to maintain the insulator or'its surroundings at a desired temperature.

There is disclosed herein a simple method of coating an insulator surface with conductive material involving repeatedly impacting the insulator surface with the con ductive material.

The prior art shows various partially successful methods of applying carbon and other conductive films. Thus, surfaces possessing suflicient inherent adherence such, for instance, as the surface of a wax object, will readily attach to itself a relatively continuous graphite layer which can be electroplated. Such films are rather delicate and usually completely opaque. Again, it is conventional to apply graphite in dispersion in fihn-forming adhesives to the surfaces of insulator objects. It has not been possible to obtain such coatings which are sufficiently conductive and, at the same time, reasonably transparent. Furthermore, there are problems connected with peeling and loose adherence of the coating and with crazing of insulator plastics to which they are applied.

A conventional device to obtain transparent conductive coatings is the application of organic films containing water-soluble electrolytes. Such films are generally very unstable as to their electrical properties and can be easily washed from the insulator base. This invention, by contrast, is concerned with obtaining a coating which is sufiiciently thin to be transparent and yet sufficiently adherent to withstand considerable abrasion and washing.

Accordingly, it is an object of this invention to provide an improved method for applying an electrically conductive coating to an insulator.

It is another object of this invention to provide a method for applying an electrically conductive coating to plastic.

A still different object is to providea glass having an electrically conductive surface.

' tent A particular object of this invention .is a method of obtaining a uniform conductive coating on a plastic surface.

Still another object of this invention is to provide a method of applying a transparent electrically conductive surface to insulators.

Another particular object of this invention is to provide a convenient method for electrically heating the surface of objects made of insulator materials.

Other objects and advantages of this: invention will appear more fully and clearly from the following description of illustrative embodiments thereof taken in connection with the appended drawings in which:

Figure 1 shows partially in section, a plan view of a typical impactor described herein.

Figure 2 is a plan view partially in section of a tumbling barrel arranged to carry out the process of this invention.

Figure 3 presents pictorially an embodiment of this invention utilizing a fluid medium to carry the impactors.

Figure 4 presents in plan a combined window and heating unit made in acordance with the process of this invention.

The process of this invention comprises repeatedly impacting the substrate to be coated with finely divided electrically conductive material. Finely divided particles, in general, lack enough mass to hit the substrate with sufii cient energy to become firmly attached to the surface. In carrying out the process of this invention the finely divided coating material is temporarily aflixed to a carrier particle having considerably greater mass, and certain other properties pointed out later, and then hurling the carrier particle at the substrate so as to utilize its kinetic energy to hammer the finely divided particles onto the substrate. The term impactor hereafter refers to the carrier.

The design of the impactor particles will depend in part on the nature and size of the surface to be coated on, the particular method by which the impactors are to be thrown against the insulator surface, and on the amount of coating which it is desired to apply In general, it is preferred that the impactor particles be round rather than angular in shape to avoid scratching of the insulator surface. Larger and heavier impactors are preferred, the harder the surface to be coated and the higher the conductivity which it is desired to impart to the treated surface.

Thus, polystyrene beads about 1 mm. in diameter have been found effective in coating polymethylmethacrylate sheet. Steel balls, inch in diameter, have been effective in applying conductive coatings to glass and have also been used to apply rather heavy conductive coatings to polymethylmethacrylate.

In general, impactors should not be smaller than 0.0025 inch nor larger than 0.25 inch, Smaller impactors lack the necessary mass to be effective. With larger impactors it is inconvenient to obtain the multitudinous impactions which are needed to secure the full advantage of this invention.

impactors may be made of any material which is stable physically and chemically under the conditions of use. They should not have such an adhesive surface as to bond the conductive material inextricably to themselves. Suitable impactor materials include glass, polystyrene, steel balls and ceramics.

The electrically conductive coating material must be finely divided and chemically stable under conditions of impact. The range of fineness is critical and the particles should not be coarser than ten microns and preferably finer than two microns in the smallest dimension. Particles of laminar habit are particularly convenient for the practice of this invention.

Impactor particles may be coated with conductive material by any of several techniques, as for instance, by exposure to the sooty products of the incomplete combustion of carbonaceous gaseous fuels or by wetting with a suspension of conductive material in a liquid followed by drying of the coated particles. The preferred method involves tumbling the impactors with finely divided conductive material until a uniform coating is obtained.

Qnly' a small amount of conductive material, usually 0.01 to 0.5% by weight of the quantity of impactor used is needed. After impactors have been used for some time it will be necessary to replenish the coating of conductive material.

I have found. that if these coated impactors are hurled at suitable types of surfaces the finely divided material will firmly adhere to the receiving surface. I have used various methods for imparting the required velocity to the impactors. These methods include use of a high velocity fluid medium such as air, a form of tumbling technique, dropping the impactors from a height above the article to be coated and the use of a vibratory mill. The use of the fluid medium and especially the tumbling type of process are preferred.

The tumbling type of operation discussed herein should not be confused with the conventional burnishing operation in which a tumbling barrel is charged with balls of ceramic or steel and objects to be burnished. The tumbling barrel is then rotated about a horizontal axis permitting the balls to tumble among the articles and polish and/ or remove the surface of the articles. Accordingly, in the burnishing operation, material may be removed from the surface of the article. Material is not added to the surface as in the process of this invention.

The typical impactor 2, shown in Figure 1, consists of a spherical head of polystyrene 4 coated with finely divided particles of carbon 6.

Figure 2 shows a tumbling barrel 8 containing an article 10 to be coated. The article 10 may be a polyniethylmethacrylate sheet. The barrel is rotated by means of driving rolls 12, 14 and the coated impactors 2 are caused to tumble and fall upon the article 10. It is preferred that the article to be coated be fixed in relation to the barrelso that uniform'coating occurs. This is particularly important in the case of irregularly shaped objects.

The manner in which the article to be treated is placed in the tumbling barrel will depend upon the shape and size of the article, the'number of articles which must be treated and on whether or not all of the surfaces of the article are to be treated.

Articles which are shaped so that they will not assume a preferential position in the barrel and which are not too large relative to the barrel may be charged loosely into the tumbling'barrel along with the impactors. For instance, I have been able to secure a uniform coating on'a piece of polymethylmethacrylate sheet,'three inches square, which was permitted to move freely in a tumbling container eight inches in diameter. A six inch square piece charged into the same container and with the same loading of impactors gave a very non-uniform coating because the piece assumed a more or less fixed position in the barrel. Since the piece spanned a very significantpart of the barrel, the impactors fell with unequal force on various parts of the piece. Consequently, various parts of the piece were unequally coated.

It is difiicult to state a general rule covering all shapes of objects but it is desirable that the greatest dimensions of a piece charged loosely to a tumbling barrel be not greater than 50%, preferably not greater than 40% of the diameter of the tumbling barrel.

A piece of irregular shape will often best be coated by jigging in a fixed position on the axis of the tumbling barrel. In this'fashion all exposed surfaces of the objects receive approximately the same degree of impact.

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it is preferred that objects be jigged in such a position that no face be perpendicular to the axis of the barrel.

Thus, it is preferred that flat shaped pieces be mounted with the plane faces parallel to the axis of the barrel. If the piece is approximately cubic it is preferred that a long diagonal of the cube be parallel to the axis of the arrci.

it is ofen the case that not all surfaces of a piece need be treated. In such cases the surfaces not be treated can be protected for instance, with masking tape. In such cases, it wiil often be advantageous to jig the articles upon the walls of the barrel with faces to be coated pointed inwards. A large sheet of plastic which is to be coated can conveniently be bent into cylindrical shape so as to form a liner or part of a liner for the tumbling barrel whereupon the impactors may be charged to the barrel and coating carried on as usual.

it has been noted that the preferred method of coating impactor with conductive materials is by tumbling. Where the tumbling barrel method of coating articles is being used, the two operations can be conducted simultaneously. Thus impactors, finely divided conductive material and articles to be coated can be charged at the same time to a tumbling barrel.

it has been found, however, that superior control over the resistivity of conductive coatings can be obtained by conducting the operations separately. This latter procedure is preferred.

It has been found that with a given amount of conductive material on the impactors, the amount of conductive material transferred to the surface under treatment approaches a stable value after a period of time. \Vith a comparatively small amount of conductive material (0.01 to 0.1 weight percent) on the impactors it has been found that this stable amount of transfer yields a transparent, static dissipating coating on polymethylmethacrylate. The coating is extremely uniform and resists considerable washing and abrasive action. The time to approach the stable condition above described is usually 4 to 12 hours.

There is shown in Figure 3 a device for hurling the coated carrier particles 2 of Figure 1 at the article to be coated. Air gun 22 utilizes a stream of air to lift carrier particles 2 from a reservoir 24 through hose 26 and hurl them at the target 10. The carrier particles are returned by gravity to the reservoir 24.

The air gun may be a conventional sand blasting gun; however, for a clear understanding of this invention it should be appreciated that in the conventional use of such apparatus the abrasive action of the missile par ticles remove material from the target surface whereas in the practice of the present invention just the opposite and hence unobvious result is obtained wherein the target surface is coated. Anobvious equivalent is to'hurl impactors at an object by centrifugal action.

For large scale production conventional shot peening equipment will be found very useful. This technique will be found to be particularly useful where the interior of a hollow shape is to be coated. Otherwise it"does not always give the high degree of uniformity which can be obtained with a tumbling barrel, as described'above. Grdinarily it is preferred to use the last mentioned method.

For a better understanding of the invention a number of specific examples are presented hereafter.

Example 1 A- 10 inch diameter jar mill of -1 gallon capacity was charged with 1000 grams of /s inch polystyrene chips, 5 grams of graphite (grade No. 8485 made by Dixon Crucible Company) and three pieces of poiymethylrneth-acrylate, 4- inches square and 4; inch thick protected on one side with masking tape. After tumbling for 5 'hours at revolutions per minute, the plastic pieces were withdrawn, washed with soap and water to remove loose graphite and examined. The treated surface was found to be uniformly coated and to have a resistivity of 0.2 megohrn per square and a light transmission of 75 Example 2 The experiment of Example 1 was repeated except that 2 grams of the same grade of graphite were used instead of grams. The treated plastic surfaces showed a resistance of 0.6 megohm per square and a light transmission of 80%.

A treated surface was rubbed vigorously with a rough cotton cloth for 30 seconds. After this treatment the resistivity was 3.5 megohms per square, still many times the minimum conductivity needed to dissipate static.

Example 3 The experiment of Example 2 was repeated except that an equal amount of grade 200- graphite made by Dixon Crucible Company was substituted for the graphite previously used. Grade ZOO-10 is considerably finer than the former material.

The treated pieces showed a resistivity of 8000 ohms,

(0.008 megohm) per square and light transmission of 55%. The experiment demonstrates the greater effectiveness of finer particles.

Example 4 The experiment of Example 3 was repeated except that 0.5 gram of the same grade of graphite was substituted for the 2 grams previously used.

The treated pieces showed a resistance of 20,000 ohms (0.02 megohm) per square and a light transmission of 65%.

Example 5 A 500 cc. bottle, 4 inches in diameter, was charged with 2000 grams of steel balls, Ms inch in diameter, and 2 grams of 200-10 grade graphite. The bottle was then rotated at 40 revolutions per minute for 5 hours. A piece of polymethylmethacrylate, l x 2" x A2", protected on one side with pressure sensitive tape, was then put in the bottle with the coated impactors and rotation was continued for 1 hour. The piece was withdrawn, washed, dried and examined. The treated surface was found to have a resistance of 0.2 megohm per square and a light transmission of 65%.

The experiment illustrates the effectiveness of the comparatively heavy steel balls.

Example 6 Coated steel ball impactors were prepared in exactly the same manner as in Example 5 except that 2.5 grams of graphite were used instead of 2 grams.

To the coated impactors was added a piece of glass, 1 x 2" x The bottle was rotated for 10 hours at 40 revolutions per minute. At the end of this time the piece of glass was removed, washed and tested. Its surface was found to have a resistance of less than 1000 megohms per square and was static dissipating. The coating was uniform in appearance and had a light transmission of 75%.

Example 7 The experiment of Example 6 was repeated exactly except that 5 grams of finely divided metallic silver was substituted for the 2.5 grams of graphite.

The treated glass showed a resistance of less than 1000 megohms per square and was static dissipating. The coating presented a uniform appearance and had a light transmission of about 30%.

Example 8 The experiment of Example 7 was repeated exactly except that 5 grams of finely divided metallic lead were substituted for the 5 grams of silver.

The treated glass surface was found to have a resistance below 1000 megohms per square and to be static dissipating. The coating transmitted about 40% of in cident light.

Example 9 Coated steel ball impactors were prepared in exactly the same manner as in Example 5 except that 5 grams of molybdenum sulfide powder were substituted for the 2 grams of graphite.

To the coated impactors were added a piece of glass and a piece of polymethylmethacrylate each 1" x 2" x As, each with only one surface expose The bottle was rotated for 10 hours at 40 revolution-s per minute. At the end of this time the pieces were removed, washed and tested.

The piece of polymethylmethacrylate and the piece of glass were both found to be static dissipating and to have resistivities below 1000 megohms per square. The piece of glass showed a light transmission of 5 0%, the piece of polymethylmethacrylatc a light transmission of 20%.

Example 10 To the jar mill of Example 1 were charged 2000 grams of XXX grade polystyrene beads made by Koppers Co., Inc., and 5 grams of grade 200-10 graphite made by Dixon Crucible Co. The beads had an average diameter of 1 mm. The mill was rotated for 5 hours at 70 R. P. M. Then, to the coated impactors were added three pieces of polymethylmethacrylate, 4 inches square and inch thick, protected on one side with pressure sensitive tape. The mill was rotated for another 105 minutes. The pieces were removed, washed, dried and tested. They were found to be all closely alike in resistance having a resistance of 4000 ohms per square and a light transmission of 55%.

Example 11 The experiment of Example 10 was repeated exactly except that 1000 grams of polystyrene beads were used instead of 2000 grams.

The coated surfaces were found to have a light transmission of 65% and a resistance of 2000 ohms per square. A piece of pressure sensitive tape was pressed firmly against the coated surface and ripped off again without appreciable change in the resistance of the coating.

Example 12 Coated impactors were prepared exactly as in Example 10 except that 0.5 gram of 200-10 grade graphite were used instead of 5 grams.

To the coated impactors were added two pieces of polymethylmeth-acrylate, one piece of polystyrene and one piece of cellulose acetate, each 3 inches square and A; inch thick. The mill was rotated at 70 revolutions per minute for 30 minutes. One piece of polymethylme'thacrylate was removed, washed, dried and tested. It was found to have a resistance of 1 megohm per square and a light transmission of 92%.

The rotation was continued for an additional 90 minutes. The three remaining pieces were removed, washed, dried and tested.

The piece of polymethylmethacrylate was found to have a resistivity of 40,000 ohms per square and a light transmission of The piece of polystyrene had a resistance of 40,000 ohms per square and a light transmission of 70%. The piece of cellulose acetate had a resistance of 30,000 ohms per square and a light transmission of 60%.

Example 13 Coated impactors were prepared exactly as in Example 3 except that 1 gram of 200-10 graphite was used instead of 2 grams. To the coated impactors was added a rod of paper-base phenolic laminate 1 inch in diameter and 3 inches long. The mill was turned at 40 revolutions .4 per minute for- 10 hours. The rod was then removed and tested. its surface was found to have a resistance of 5000 ohms per square. Various parts of the surface were isolated and tested separately. The uniformity of resistance was found to be excellent.

The coating produced by the process of this invention is extremely tenacious as may be judged from an experiment wherein a 3 x 3" x /s sheet of polymethylmethacrylate bearing a coating made as in Example 12 was subjected to test. The resistance of the coating was measured from corner to corner. Then the sheet was heated to 250 F. and bent, on an axis parallel to one side, about 1 /2" diameter mandrel. The sheet was straightened and a second fold made at right angles to the first. Upon restoring the sheet to its original fiat shape it was found that the piece had not lost its conductivity.

Exan'zple 14 Impactors coated as in Example 10 were sent through a sand blast gun operating at 96 pounds pressure and directed at a sheet or" polymethylmethacrylate. The impacting was continued until the light transmission was 75%. The resistance of the piece was 50,360 ohms square. The coating was adherent as shown by the previously described pressure sensitive tape test.

In Figure 4 there is shown a heating element made in accordance with the method of this invention. A complex shaped plastic window 52, such as may be used in aircraft Windshields, is coated on one face with finely divided carbon so as to provide an electrically conductive transparent surface.

This invention permits many variations in processing; for example, making tape may be applied to selected portions of the article prior to coating with conductive material so that after processing, sharply defined regions of non-conductivity exist. Other techiques such as buffing or rubbing the conductive material into the surface tends to force the material under the edges of the masking whereas impacting does not.

By masking olf areas such as region with masking tape prior to the impacting the conductive paths between electrodes 46, 4.3 are equalized so as to provide uniform heating over the entire surface. A source of potential 50 is shown connected to electrodes 46, 43.

Example I 5 A sheet of polymethylmethacrylate, 4 x 4 x was coated on one face as in Example 10. Electrodes were pointed along opposite edges of the coated face. A source of electrical energy was connected to the electrodes. it was found that at 260- volts, watts of energy. was dissipated. This is quite sufficient to provide a useful heating element.

in view of the surprising phenomenon pointed out in the preceding examples, wherein impactors were used to coat an object, the following example is of interest.

Example 16 The coated panel of Example 3 was tumbled in the same container together with the identical amount of the impactors. The impactors in this example differed in that they were not coated. After 1 hour of tumbling it was found that the resistance of the coating had increased to 0.6 megohm per square.

Although it hasbeen pointed out that adhesives are not required in carrying out this invention it is to be noted that pretreatment of the surface before coating may be helpful under some circumstances. Such treatment may include the cleaning of the surface or the heating of the surface until it is tacky.

In carrying out the invention the atmosphere within thetumbling barrel may be adjusted to meet the'requirements of the particular combination of the coating material and article to be coated such, as hot or cold temperature ranges and/or a reducing gas atmosphere to prevent oxidation of the various surfaces.

Graphite compacts or chips may be used as impactors which are capable of transferring fine particles of their own substance to a surface which they impact.

The process of this invention may be applied to the coating of metallic surfaces or elastomeric (e. g., rubber) surfaces.

Although preferred embodiments have been disclosed, it will be understood that modifications may be made within the spirit and scope of the invention.

I claim:

1. The process of rendering the surface of solid insulator articles electrically conductive comprising hammering said. surfaces with impactors substantially smaller than said articles but at least 0.0025 inch in size, said iinpactors having at least a portion of their surface coated with finely divided electrically conductive material having a particle size less than 10 microns wherein said impactors are conveyed by a gaseous stream to the said article to be coated.

2. The process of claim. 1 wherein said impactors are between 9.0025 inch and 9.25 inchin size.

3. The process of claim 1 wherein said articles are formed of a transparent synthetic resin and said electrically conductive material is a carbon.

4. The method of applying an electrically conductive transparent coating of finely divided solid non-fusible conductive material having an average particle size of less than 10 microns, to an electrically non-conductive thermoplastic substrate comprising the steps of repeatedly impacting said substrate with impactors substantially smaller than said substrate but at least 0.0025 inch in size, coated with said finely divided solid nonfusible conductive material, and stopping said impacting when said substrate has deposited thereupon a thin transparent electrically conductive coating of said finely divided solid non-fusible conductive material.

5. The method of claim 4 wlerein said substrate is transparent.

6. The method of claim 4 wherein said substrate is polymethylrnethacrylate.

7. The method of claim 4- wherein said impactors have a particle size between 0.0025 inch and 0.25 inch.

8. The method of claim 4 wherein said finely divided non-fusible conductive material is a carbon.

9. The process of claim 4 wherein said iinpactor elements are steel balls.

10. The process of claim 4 wherein said electrically conductive material is a metal powder having a particle size less than ten microns.

ll. The process of claim 4 wherein said electrically conductive material is a metal.

12. The process of claim 4 wherein the step of impact ing is carried out in a tumbling barrel.

References Cited in the file of this patent Nov. 15, 1947.

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1. THE PROCESS OF RENDERING THE SURFACE OF SOLID INSUIATOR ARTICLES ELECTRICALLY CONDUCTIVE COMPRISING HAMMERING SAID SURFACES WITH IMPACTORS SUBSTANTIALLY SMALLER THAN SAID ARTICLES BUT AT LEAST 0.0025 INCH IN SIZE, SAID IMPACTORS HAVING AT LEAST A PORTION OF THEIR SURFACE COATED WITH FINELY DIVIDED ELECTRICALLY CONDUCTIVE MATERIAL HAVING A PARTICLE SIZE LESS THAN 10 MICRONS WHEREIN SAID IMPACTORS ARE CONVEYED BY A GASEOUS STREAM TO THE SAID ARTICLE TO BE COATED. 